This application is based on and claims priority from Japanese Patent Application Nos. 2000-218280, 2000-238300 and 2001-167540 filed on Jul. 19, 2000, Aug. 7, 2000 and Jun. 4, 2001, respectively, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a fuel cell apparatus or a fuel cell stack.
2. Description of Related Art
A PEFC (Polymer Electrolyte Fuel Cell) apparatus includes a stack of fuel cells. Each fuel cell includes a membrane-electrode assembly (MEA) and a separator. The MEA includes an electrolyte membrane and a pair of electrodes disposed on opposite sides of the electrolyte membrane. The pair of electrodes include a fuel electrode (anode) constructed of a first catalyst layer with a first diffusion layer and an oxidant electrode (cathode) constructed of a second catalyst layer with a second diffusion layer. The separator has a passage formed therein for supplying fuel gas (hydrogen) to the anode and for supplying oxidant gas (oxygen, usually, air) to the cathode. A plurality of fuel cells are formed to construct a piled module. Electrical terminals, electrical insulators, and end plates are disposed at opposite ends of the pile of modules to construct a stack of fuel cells. After tightening the stack of fuel cells between the opposite end plates in a fuel cell stacking direction, the end plates are coupled to the fastening member (or a tension plate) extending in a fuel cell stacking direction outside the pile of fuel cells by bolts extending perpendicularly to the fuel cell stacking direction.
In the PEFC, at the anode, hydrogen is changed to positively charged hydrogen ions and electrons. The hydrogen ions move through the electrolyte to the cathode where the hydrogen ions react with oxygen supplied and electrons (which are generated at an anode of the adjacent MEA and move to the cathode of the instant MEA through a separator) to form water as follows:                     At  the  anode:                                      H          2                →                              2            ⁢                          H              +                                +                      2            ⁢                          e              -                                                              At  the  cathode:                                                  2            ⁢                          H              +                                +                      2            ⁢                          e              -                                +                                    (                              1                /                2                            )                        ⁢                          O              2                                      →                              H            2                    ⁢          O                    
A cooling passage is formed at all of the modules, so that the fuel cell apparatus is cooled by a coolant (usually, water) flowing through the cooling passage. As a result, the temperature of the fuel cells is controlled by the temperature between the environmental temperature (about 20xc2x0 C.) and the operating temperature (about 80xc2x0 C.).
In order that the above electrical-chemical reaction is normally conducted, pressure acting on the stack of fuel cells is required to be evenly distributed and be maintained constant over a whole fuel cell face despite the above changing temperature.
Japanese Patent Publication HEI 9-259916 discloses a structure for tightening the stack of fuel cells using four rods extending outside of the stack of fuel cells. Nuts are then threaded to the end of the rods in order to tighten the stack of fuel cells, in order to provide evenly distributed pressure. A coil spring is disposed between the nut and the stack of fuel cells, whereby a variance of the load is decreased.
However, with the conventional tightening structure, there is a problem that it is difficult to tighten the stack of fuel cells with an evenly distributed pressure if the fuel cells are not parallel with each other, resulting that the power characteristic of the fuel cell decreases and in a worst case, leakage of the reactant gas (hydrogen, air) occurs. Further, with the tightening structure using four rods, the rod further extends outwardly from the end plate, and the fuel cell apparatus is too long so that the mounting of the fuel cell apparatus to a vehicle is not easy.
An object of the present invention is to provide a fuel cell apparatus which can tighten a stack of fuel cells in a fuel cell stacking direction with an evenly distributed pressure.
Another object of the present invention is to provide a fuel cell apparatus with improved attaching capabilities to a vehicle.
A fuel cell apparatus according to the present invention includes a stack of fuel cells having a fuel cell stacking direction and a first end and a second, opposite end in the fuel cell stacking direction. First and second end plates are disposed on the first end and the second end of the stack of fuel cells. The first and second end plates pressing the stack of fuel cells therebetween, and are coupled to a fastening member extending in the fuel cell stacking direction outside the stack of fuel cells. The first end plate having an inboard surface facing said stack of fuel cells. A pressure plate disposed on a side of the first end of the stack of fuel cells and inboard of the first end plate, the pressure plate having an outboard surface facing the first end plate. The first end plate has a concave portion formed in the first end plate at the inboard surface of the first end plate. The pressure plate has a convex portion having a curved surface formed in the pressure plate at the outboard surface of the pressure plate. The convex portion contacts the concave portion.
Each of the first and second end plates may be coupled to the fastening member by a serration and a bolt. The first end plate may include an end plate main portion and an adjusting portion adjustable in a position relative to the end plate main portion in the fuel cell stacking direction. The concave portion may be formed in the adjusting portion.
A load variance decreasing mechanism may be disposed at at least one position of in the first end plate, in the pressure plate, and between the pressure plate and the first end plate, and in series with contact portion of the convex portion with the concave portion in a tightening force transmitting direction.
An electrical insulator may be disposed inboard of the pressure plate. The electrical insulator may have an outboard surface and a recess formed in the electrical insulator at the outboard surface of the electrical insulator. The pressure plate may be disposed in the recess of the electrical insulator.
The curved surface of the convex portion may comprise a spherical surface. In a case where the fuel cells are restricted in dislocation in one of two directions perpendicular to the fuel cell stacking direction by the fastening member, the curved surface of the convex portion may comprise a cylindrical surface curved in the other direction of the two directions in which the fuel cells are not restricted in dislocation by the fastening member.
The load variance decreasing mechanism may comprise a plurality of sets of coned disk springs, disposed in series with each other. The first end plate may include an end plate main portion and an adjusting portion adjustable in position relative to the end plate main portion in the fuel cell stacking direction. At least one portion of the load variance decreasing mechanism may be disposed between the end plate main portion and the adjusting portion. The adjusting portion may include a female thread portion restricted in rotation relative to the end plate main portion and a male thread portion threaded to the female thread portion and adjustable in position relative to the female thread portion in an axial direction of the male thread portion.
The pressure plate may be divided into two members in the fuel cell stacking direction, and at least one portion of the load variance decreasing mechanism may be disposed between the two members of the pressure plate.
The pressure plate may be divided into two members including an outboard member and an inboard member in the fuel cell stacking direction. The outboard member has the convex portion formed therein and a load sensor provided therein. The pressure plate may include an outside surface having a height in the fuel cell stacking direction smaller than a height of an inside surface in the fuel cell stacking direction of the recess formed in the electrical insulator.
The load variance decreasing mechanism may include at least one set of coned disk springs which is reversed in taper angle when a fuel cell stacking force acts on the at least one set of coned disk springs.
At least one pair of spring seats may be provided to the pressure plate and the end plate, for contacting and supporting the at least one set of coned disk springs at a radially inner end and a radially outer end of the at least one set of coned disk springs. Each pair of spring seats may be inclined at an angle equal to or greater than the taper angle of corresponding set of coned disk springs reversed in taper angle.
The fuel cell apparatus may include an attaching member for attaching the fuel cell apparatus to a vehicle to which the fuel cell apparatus is mounted. The attaching member may be constructed of a portion of the fuel cell apparatus itself. The attaching member may be the fastening member connecting the first and second end plates. The attaching member may have a attaching portion protruding in a direction away from the stack of fuel cells and in a direction perpendicular to the fuel cell stacking direction. The attaching member may be attached to the vehicle at the attaching portion.
The stack of fuel cells may have a recess receding from a surface of the stack of fuel cells in a direction perpendicular to the fuel cell stacking direction, and the attaching member may have an attaching portion at a position corresponding to the recess. The attaching member is attached to the vehicle at the attaching portion. The attaching member may be any one of the first and second end plates and a separator of a fuel cell. The attaching member may be a member for supplying reactant gas or coolant to the fuel cell apparatus.
With the above fuel cell apparatus according to the present invention, since the end plate and the pressure plate is pressed to each other at the contact portion of the convex portion and the concave portion, even if the fuel cells are not parallel with each other, the end plate can press at a point the pressure plate at the contact portion of the convex portion and the concave portion so that the pressure plate can press the stack of the fuel cells at an evenly distributed pressure over the entire transverse cross-sectional area of the stack of fuel cells. Further, since the convex portion is formed in the pressure plate, the variance in parallel between fuel cells can be absorbed by a rotation or tilting of the pressure plate about the center of curvature of the curved surface of the convex portion without being accompanied by dislocation of the fuel cells in a direction perpendicular to the fuel cell stacking direction. Further, since the concave portion is formed in the end plate, the convex portion and the concave portion do not dislocate relative to each other in the transverse direction perpendicular to the fuel cell stack direction and the coupling is stable.
Further, in a case where the stack attaching member is constructed of a portion of the fuel cell stack itself, it is not necessary to provide an extra attaching member used only for attaching the stack of fuel cells to a vehicle, so that attaching the stack of fuel cells to the vehicle by a small number of attaching members becomes possible.